The goal of this proposal is to optimize engineered intracellular antibodies (intrabodies) as novel clinical reagents and drug discovery tools for the treatment of Huntington's Disease (HD), with broad, long-term relevance to other neurodegenerative disorders caused by misfolded proteins. Intrabodies use the target specificity of antibodies to form complexes with intracellular proteins, and are already in clinical trials for treatment of cancers and AIDS. The research design starts with in vivo testing with a single-chain Fv anti-huntingtin (htt) intrabody (scFv C4) that has shown significant rescue of HD phenotypes in cell lines, organotypic slice cultures and a Drosophila HD model;plus a newer single domain intrabody (VL 12.3)that shows even stronger anti-htt aggregation properties in situ. Delivery of the intrabody genes will utilize a non-primate lentivirus, Equine Infectious Anemia Virus (EIAV), with either a VSVG or Rabies-g envelope, as one gene therapy vector, with some experiments to compare with delivery using AAV vectors provided by a collaborator. Quantitative assays of abnormal nuclear htt accumulation and aggregation, DARPP- 32 levels, and open field activity behavior will be used to assess the efficacy of the intrabodies delivered to the brains of Exon 1 transgenic (R6/1) and Hdh knock-in (Q111) mouse models on the same inbred genetic background. Simultaneously, screening and testing of a small pool of newer intrabodies will be done using anti-aggregation, protection, and toxicity assays in neuronal cell lines. The most successful of the new intrabodies will then be tested as above. If correction is incomplete with individual intrabodies, combination therapies will be tested in cells and in vivo. At the end of these studies, we will have established the optimal characteristics of intrabodies for eventual HD therapeutics and further drug discovery. These approaches should also be generally applicable for other neurodegenerative diseases that result from abnormal protein folding and accumulation, including Alzheimer's, Parkinson's, and prion diseases.